This invention relates to an apparatus for measuring a profile of an electron beam projected onto a display surface of a cathode ray tube (hereinafter, referred to as "CRT") equipped with a shadow mask, or a distribution of electron energy density on a CRT display surface.
Conventionally, a method for measuring the profile of an electron beam has been proposed as a method for highly accurately and quantitatively evaluating the focusing performance of a color CRT.
This method for measuring the profile of an electron beam is basically such that electron beams having a specified electron energy are projected onto phosphors, thereby converting them into light energy and this light energy is sensed by being photoelectrically converted into an electric energy by an image pickup device such as a CCD (Charge Coupled Device), and the profile of the electron beam is quantitatively measured using pixel data obtained by the image pickup device.
However, in the color CRT equipped with a shadow mask, phosphors of three primary colors of red (R), green (G), blue (B) are discretely and regularly applied to the display surface, and only parts of the electron beams are caused to transmit by the shadow mask to make specified phosphors luminous. Accordingly, even if a spot of electron beam is projected in a specific position of the CRT display surface, thereby making a phosphor luminous so that this phosphor is sensed by the image pickup device, a data (luminance information in a plurality of positions within the section of the beam) cannot be obtained in sufficient quantity to specify the profile of the electron beam.
In view of the above problem, there has been proposed a method according to which phosphors are made luminous in a plurality of positions within the section of an electron beam quantitatively measuring the profile of the electron beam using pixel data obtained by sensing the luminous phosphors by the image pickup device.
For example, U.S. Pat. No. 4,408,163 discloses an electron beam profile measuring apparatus in which the position of a projected electron on a single specific phosphor is finely displaced along vertical and horizontal directions, the luminous phosphor is sensed in the respective projection positions, and the profile of the electron beam is calculated using the relative luminous position and luminance of the phosphors within the electron beam in the respective projection positions.
Further, Japanese Unexamined Patent Publication No. 8-203436 discloses an electron beam profile measuring apparatus in which the positions of a projected electron beam on a plurality of specific phosphors are finely displaced along vertical and horizontal directions, the plurality of luminous phosphors are sensed in each projected position, and the profile of the electron beam is calculated using the relative luminous positions and luminances of the plurality of phosphors within the electron beam in the respective projection positions.
The above prior art measuring apparatus disadvantageously requires a long time for the measurement since the data necessary for the calculation of the profile of the electron beam is obtained by finely displacing the projection position of the single electron beam along vertical and horizontal directions with respect to the specific phosphors and repeatedly sensing the phosphor in each projection position.
Assuming, for example, that the sectional profile of the electron beam on the CRT display surface is a circle of the diameter of 1 mm, resolving power in the measurement of the profile of the beam is 40 .mu.m, and the interval of the phosphors of the CRT equipped with a shadow mask is 0.28 mm, it is necessary to repeat the sensing about 625 times (=1/(0.04).sup.2) by finely displacing the electron beam along vertical and horizontal directions with respect to the specific phosphors in the measuring apparatus of U.S. Pat. No. 4,408,163 while it is necessary to repeat the sensing about 49 times (=0.28/(0.04).sup.2) by finely displacing the electron beam along vertical and horizontal directions with respect to the specific group of phosphors in the measuring apparatus of Japanese Unexamined Patent Publication No. 8-203436.
In order to repeat the sensing a plurality of times, high stability is required during the measurement. For example, the blurring of the sensed image due to a vibration during the measurement may cause a measurement error.